1. Field of the Invention
The present invention relates to half-mirrors. More specifically, it relates to a half-mirror usable over a wide wavelength region including the visible and infrared spectrum.
2. Related Art
The present specification uses the following symbols as defined below, as commonly used by those skilled in the art:
n means an index of refraction; PA1 n.sub.sub means the index of refraction of a substrate (such as glass), and is also used loosely to denote the substrate itself; PA1 n.sub.H means an index of refraction higher than n.sub.sub, and is also used loosely to denote the substance itself having that index of refraction; PA1 n.sub.L means an index of refraction lower than n.sub.sub, and is also used loosely to denote the substance itself having that index of refraction; PA1 1, 2, 3, . . . N-1, N are subscripts denoting layer numbers of dielectric substances. PA1 TiO.sub.2 (refraction index n.sub.H =2.35) is used as the film n.sub.H ; PA1 MgF.sub.2 (refraction index n.sub.L =1.38) is used as the film n.sub.L ; PA1 each film has an optical path length (or layer thickness) of .lambda..sub.o /4 (where .lambda..sub.o =center wavelength); PA1 the superscript 2 in (n.sub.H .multidot.n.sub.L).sup.2 simply means a repetition of the (n.sub.H .multidot.n.sub.L) layers which are in the parentheses; PA1 "n.sub.sub /" at the beginning of the definition means that the substrate is present on a first side of the layers; and PA1 "/air" at the end of the definition means that air is present on the opposite side of the layers than the substrate. PA1 (1) a substrate having index of refraction n.sub.sub ; and PA1 (2) layers with indices of refraction (n.sub.H .multidot.n.sub.L).sup.2 of the known four-film type dielectric half mirror having a construction represented by n.sub.sub /(n.sub.H .multidot.n.sub.L).sup.2 /air; PA1 (a) high refractive index dielectric substances n.sub.H1, n.sub.H2 . . . ; and PA1 (b) low refractive index dielectric substances n.sub.L1, n.sub.L2 . . . ,
Further, as used in this specification to describe the invention, coefficients such as 1 and 2, when preceding the characters n, indicate that the corresponding dielectric layers have thicknesses of .lambda..sub.o /4 and .lambda..sub.o /2, respectively. Thus, 1n.sub.L.sbsb.-- (or simply, n.sub.L.sbsb.--) denotes a substance having an index of refraction n.sub.L and a thickness of .lambda..sub.o /4. In contrast, 2n.sub.H.sbsb.-- denotes a substance having an index of refraction n.sub.H and a thickness of .lambda..sub.o /2. When the coefficient is 1.1, 2.1 or 2.2, and the angle of incidence is 45.degree., the invention provides that thicknesses are corrected to be 1.1(.lambda..sub.o /4), 2.1(.lambda..sub.o /4), 2.2(.lambda..sub.o /4).
A first known half-mirror for the visible wavelength region is, for example, a four-layered half-mirror illustrated in FIG. 12. This known half-mirror has a construction which may be represented by: EQU n.sub.sub /(n.sub.H .multidot.n.sub.L).sup.2 /air
in which:
FIGS. 9 and 10 show the spectral transmission/reflection characteristics of the known half mirror shown in FIG. 12. FIG. 9 shows characteristics obtained in response to incident light applied perpendicular to the mirror (0.degree., by convention), while FIG. 10 shows the characteristic obtained when the incident angle is 45.sub.o from the perpendicular. Solid-line curves T and broken-line curves R show transmissivity and reflectivity, respectively.
In all drawing figures, when the incident angle is 45.degree., the film thicknesses are adjusted for use at that incident angle. Furthermore, when the incident angle is 45.degree., the reflectivity and the transmissivity are shown as mean values of the P and S components of the polarized light. As described in Chapter 25 of Fundamentals of Optics, Jenkins et al. (McGraw-Hill; Kogakusha), incorporated herein by reference, the P "vibrations" are those parallel to the plane of incidence, and the S "vibrations" are perpendicular to the plane of incidence.
A second known half-mirror is capable of covering a wide wavelength region including visible and infrared regions, such as a half-mirror obtained by forming a thin metal film such as CHROMEL (nickel-chrome, e.g., NiCr 80:20) coated on a transparent substrate. The spectral transmission/reflection characteristics of such a half-mirror are shown in FIG. 11.
As apparent from the spectral characteristics in FIGS. 9 and 10, the first known type dielectric half-mirror has a transmitted:reflected light quantity ratio of 1:1 in the visible region, as is desirable. However, in the near-infrared region of wavelength region (between 700 to 900 nm), the light-quantity ratio of 1:1 cannot be obtained because transmissivity increases and reflectivity decreases, with increasing wavelength. Thus, the wavelength region over which the half-mirror functions is narrowed, which is undesirable.
The second type of half-mirror, such as that illustrated in FIG. 11, exhibits flat spectral characteristics over a wide wavelength region. However, half-mirrors of this type also exhibit a large light absorption, about 40%. Hence, such half-mirrors cause a large loss of light quantity, which is also undesirable.
Thus, these known half-mirrors have either a disadvantage of either too narrow a bandwidth or too much light absorption. There is a need in the art to provide a half-mirror which both has both a wide bandwidth and which has low light absorption.